The barrel when first measured would have been between .563 and .571 so marked correctly as 26 bore. The chamber size is marked in the lozenge mark as 28 which is the cartridge it is designed for. I think the 20 with the overstamp will have been a mistake then rectified by someone picking up the wrong number punch. It does happen occasionally and not all that uncommon for marks to be missed off or a wrong one added and struck out when noticed. There are a high volume of guns going through the Proof House daily and human error will be encountered. It was marked for black powder initially on the underside of the barrel and later re-proofed for nitro. If in doubt send photos to the Birmingham Proof House and they will advise. It's what they're there for and always ready to help. Lagopus…..
